Conventionally, entire image data undergoes encryption, scrambling, or the like to transfer image data or the like in secrecy. This is a technique for encrypting entire image data using an encryption key, and allowing only a party who has a decryption key corresponding to the encryption key to normally decrypt the encrypted image data.
Especially, image data having a hierarchical structure undergoes an encryption process using different encryption keys for respective layers for the purpose of controlling reproduction of the image data in correspondence with the hierarchical structure. Also, image data made up of a plurality of tiles undergoes an encryption process using different encryption keys for respective tiles for the purpose of controlling reproduction for respective tiles. Furthermore, when image data is made up of a plurality of tiles, and each tile has a hierarchical structure, the image data undergoes an encryption process using different encryption keys for respective layers in each tile for the purpose of controlling reproduction of the image data in correspondence with each tile and hierarchical structure.
When image data is encrypted using different encryption keys for respective tiles and layers to be controlled, reproduction of the image data can be controlled for each tile and hierarchical structure. However, in order to decrypt a predetermined tile and layer of encrypted image data, all the encryption keys used in the encryption process must be managed, and an appropriate decryption key must be supplied upon decryption.
When reproduction of a predetermined tile and layer of the encrypted image data is permitted, unpermitted tiles and layers of the image data remain encrypted. Such image data with the encrypted tiles and layers is reproduced like a noise signal, and a problem is often posed.
For example, a case will be examined wherein image data having a hierarchical structure associated with resolutions undergoes an encryption process using different keys for low- and high-resolution components, and image reproduction control corresponding to a resolution is made. When it is permitted to browse a low-resolution image, but it is not permitted to browse a high-resolution image, the low-resolution components are decrypted using the key corresponding to the low-resolution components, and the high-resolution components remain encrypted. In such case, image data obtained by multiplexing noise data of the high-resolution component on the image data of the low-resolution components is reproduced, and the low-resolution image cannot be browsed without being multiplexed with the noise data.
When image data is encrypted using different encryption keys for respective tiles and layers, there is no means for determining correspondence between encrypted predetermined tiles and layers, and decryption keys required to decrypt them. For this reason, a decryption process cannot often be normally made. Likewise, when some tiles or layers of image data are encrypted, it is difficult to discriminate encrypted tiles and layers from those which are not encrypted, and a decryption process cannot often be normally made.
The present invention has been made in consideration of the above problems, and has as its object to allow easy management of keys used in encryption.